Magnetization and electron paramagnetic resonance of Co clusters embedded in Ag nanoparticles

We report magnetization and electron paramagnetic resonance (EPR) measurements on Co clusters embedded in Ag obtained by an inverse micellar technique. The cluster size (approx =10 atoms), the saturation magnetization (M sub s ), and the anisotropy constant were obtained from magnetization measurements at low temperature. In the as-prepared sample we found that M sub s /M sub s exp bulk = 0.44 plus/minus 0.05. Reduction of the sample under a H sub 2 flux at 373 K leads to an enhancement of the ratio M sub s /M sub s exp bulk to 1.16 plus/minus 0.19 - consistent with Co-cluster-beam experiments - and to an anisotropy constant of K = (8.5 plus/minus 6.5) mult 10 exp 7 erg cm exp -3 , much larger than the corresponding bulk value. The EPR clearly shows the effect of dynamic narrowing due to superparamagnetic relaxation. Modelling the EPR spectra, we obtained independently the anisotropy energy value and the volume dispersion.